1. Field of the Invention
The present invention relates to a pattern forming method, a pattern designing method, and a mask set.
2. Description of the Related Art
In a method of manufacturing a semiconductor device in the past, to form a plurality of device patterns on a semiconductor wafer of silicon or the like, a large number of different mask patterns are sequentially laid one on top of another on the semiconductor wafer and exposed to light. In the exposure, an exposure device positions respective masks using an alignment mark. In a state in which the masks are positioned, the exposure device performs overlay shift inspection for inspecting whether a device pattern that should be formed next is formed to be correctly stacked on a device pattern on each chip already provided on the semiconductor wafer.
There is a scatterometry system as a system for the overlay shift inspection. In the scatterometry system, light is irradiated on marks including diffraction gratings respectively arranged and formed in a first layer in which a first device pattern is formed and a second layer formed on the first layer and including photoresist exposed and developed into a second device pattern. Subsequently, diffracted light in marks of the repeated patterns is detected, whereby a sectional profile corresponding to the marks of the repeated patterns is calculated and an amount of overlay shift is determined. A first mark formed in the first layer and a second mark formed in the second layer are formed to partially overlap (see, for example, US2008/0144036A1).
As the marks for the overlay shift inspection, first, in the first layer, the first mark including recesses having a predetermined period is formed on, for example, a dicing line between chips together with the first device pattern by using the photolithography technique and the etching technique. Thereafter, photoresist is applied on the first layer to form the second layer. The second mark including recesses having a predetermined period is formed, together with the second device pattern, on a dicing line to partially overlap the first mark. When the second mark is formed, a step is formed on an upper surface of the photoresist according to the first mark. Therefore, the second mark is affected by defocus during exposure in a lithography process. As a result, it is likely that facon of the marks is spoiled to cause deterioration in measuring accuracy.